1. Field of the Invention
This invention pertains generally to circuit board fabrication, and more particularly to copper and steel components for use in the manufacture of printed circuit boards.
2. Description of the Background Art
In early stages of manufacturing technology, printed circuit board (PCB) lay-up panels were laminated using presses similar to those used in the wood industry for laminating, for example, sheets of plywood. Hydraulic-driven presses were used, and steam or electric power was used to heat the presses to temperatures exceeding 350.degree. F. The panel components in the presses were submitted to pressures between 300 psi and 500 psi at 350.degree. F. for approximately one hour to achieve lamination. A typical multilayer layup configuration 10 of two PCB panels 12a, 12b in a press opening is schematically shown in FIG. 1. A highly polished and precision ground stainless steel plate 14 approximately 0.062 inches thick was used to separate each panel 12a, 12b within the press opening, and like 0.062 inch stainless steel plates 16a, 16b were placed on the top and bottom of the stack, respectively. In the layup, a first thin copper layer 18a would be placed on the separator sheet 16a with its working surface facing the separator sheet. A laminated multilayer core 20a would be placed over copper layer 18a. While core 20a is shown as a single layer for simplicity, core 20a would typically comprise a multilayer assembly of a plurality of plys of prepreg, a plurality of double sided boards, and conductive paths between the boards. A second thin copper layer 18b would be placed over core 20a, with its working surface facing away from the core. Stainless steel separator plate 14 would then be placed over copper layer 18b, and a second book, comprising an identical arrangement of a core 20b between copper layers 22a, 22b would be layed up on separator plate 14.
Typically, a T-304 full hard alloy or equivalent material was used for the aforementioned 0.062 inch stainless steel separator plates. A problem, however, was that the 0.062 inch stainless steel separator plates required cleaning or scrubbing to remove debris after every use and periodically needed to be resurfaced to remove dents and scratches due to handling and use. Eventually, the plates had to be replaced.
During the late 1980's, the introduction of vacuum assisted presses permitted the use of lower pressures during the lamination cycle. The pressures used in vacuum assisted presses typically ranged from approximately 150 psi to 250 psi, as opposed to the 300 psi to 500 psi range used in the hydraulic steam driven or electric presses. With vacuum assisted presses, aluminum separator sheets ranging in thickness from 0.007 inches to 0.015 inches were tested and then used extensively. Test results published during that time indicated that thin aluminum separator plates far exceeded the performance of steel plates for laminating PCB panels. These thin aluminum separator sheets were discarded after the lamination process, thus eliminating the need for expensive steel plate cleaning and handling operations and the frequent and high capital investment needed to replace the steel plates. The alloy used for aluminum separator plates is typically 3000 series (e.g. 3003, 3004, 3105 or equivalent) with a H19 hardness designation, which is identical to the alloy used to make aluminum beverage cans. The process using thin aluminum separator sheets along with low pressure from vacuum assisted presses has worked well for typical 4 layer to 6 layer PCB's with circuit lines of approximately 0.008 inches in width and approximately 0.008 inches apart. A typical configuration 24 in a press opening is schematically shown in FIG. 2 where 0.062 inch stainless steel plates 16a, 16b were placed on the bottom and top of the stack, respectively, and a thin aluminum sheet 26 was used to separate each PCB panel. The rate of production in these vacuum assisted presses increased to about 10-14 PCB panels per typical 11/2 inch press opening compared to the 6 to 8 PCB panels achieved using 0.062 inch stainless steel sheets between the books as shown in FIG. 1.
Technological advancements, however, have driven a need for PCBs having more and denser circuitry. This means that circuits must have finer lines (less than 0.006 inches wide) and closer spacing between circuit lines (less than 0.006 inches). Denser surfaces on a PCB permit a higher quantity of electrical components to be mounted thereon, thus enabling faster information processing and greater miniaturization of electronic hardware. These greater technological demands have made the surface quality of the laminated circuit board panels even more critical. Problems such as surface roughness and image transfer that also previously existed, have now become critical issues that require resolution, as any minute bump on the surface of the aluminum sheet will be transferred to the top surface of the board necessitating scraping the board and reworking the PCB fabrication process.
To prevent and minimize scrap and rework due to image transfer and surface quality problems, almost every press configuration used today employs 0.062 inch stainless steel plates (usually T-304 or T-600 stainless steel) placed adjacent to the thin aluminum separator sheets in addition to on the top and bottom of the stack. Many press loads have at least three 0.062 stainless steel plates added to the lay-up, which then reduces the number of panels that can be laminated in each press cycle. Some of the lay-up configurations have both aluminum sheets and 0.062 inch stainless steel plates separating every panel in the press, with the aluminum separator sheets being discarded after the press cycle. This approach, however, has not completely cured the problem as it causes a decrease in the production rate of the press. Also, pits, dents and other surface imperfections due to the re-introduction of steel plates into the process are still causing scrap and rework of PCB panels. Moreover, many PCB fabricators have to purchase additional new 0.062 inch stainless steel plates and again install expensive plate cleaning and handling systems. Although the thin aluminum separator sheets are discarded after every press cycle, the steel plates must be cleaned before each use, adding additional operational steps and cost to the PCB fabrication process. To maintain production demands, fabricators must purchase additional vacuum presses, at a cost of approximately $250,000 to $1,000,000 per unit, to compensate for the loss of productivity due to the re-introduction of steel plates into the PCB fabrication process.
Today, fabricators are producing between 3 and 8 PCB panels on high technology dense boards with more quality problems and at a high cost. Dense state of the art PCB's now require 2 separators, an 0.062 inch stainless steel plate and a thin sheet of aluminum. This is an expensive step backward to the beginning of the evolution of the PCB fabrication process.
Furthermore, use of a thin piece of aluminum in a copper/aluminum or a thin piece of stainless steel in a copper/stainless steel laminate structure, which have also been used for manufacturing PCBs instead of using separate layers of copper and aluminum or copper and stainless steel, do not meet today's demanding requirements for high technology, dense PCB's. Copper/aluminum laminates suffer from a number of drawbacks which include susceptibility to print through and image transfer, misregistration, blistering, warpage and delamination. Copper/stainless steel laminates eliminate some of these problems but, like aluminum, tend to exhibit unacceptable surface roughness.
Off contact printing often results from image transfer. This generally inhibits the adhesion of dry film and the ability to expose a one to one image on panels. As a result, copper/aluminum laminates are typically limited to fabricating four to six layer PCB's. In addition, shims are often required between every PCB panel. The use of shims adds significant cost to PCB manufacture. The shims must go through a labor intensive cleaning process between each use. Shims are very expensive and many PCB manufacturers have had to set aside space in their manufacturing facilities for shim cleaning.
Misregistration results from too much movement in the inner layers. This causes drill breakage and renders the PCB useless. Drill breakage also results from misregistration in high technology PCBs where small holes which are less 13 mils and as small as 4 mils are typical.
Blistering results from the uneven coefficient of thermal expansion exhibited by aluminum. The uneven CTE creates more hot spots which cause blistering. This problem may not even show up for six months or more after fabrication and, therefore, may cause major system failures.
Surface roughness is also a problem with aluminum. The high surface roughness will cause off contact printing, broken drill bits, and loss of materials. The laminates are also susceptible to warpage which renders them useless. And, delamination has been observed using the laminates at low pressures.
Therefore, a need exists for a metal separator sheet that eliminates the need for use of conventional 0.062 stainless steel separator plates while solving the problems associated with aluminum and stainless steel separator sheets. The present invention meets the foregoing needs, as well as others, and overcomes the deficiencies found with conventional layup techniques.